my_list = [1, 3, 5, 7, 9, 11, 12, 45, 77, 87,123,12321]

def binary_search(list, item):
    low = 0
    high = len(list) - 1

    while low <= high:
        mid = (low + high)//2
        guess = list[mid]
        if guess == item:
            return mid;
        if guess > item:
            high = mid - 1
        else:
            low = mid + 1

    return None

print(binary_search(my_list, 3))
print(binary_search(my_list, -1))
print(binary_search(my_list, 77))


